This invention relates to the separation of vanadium values from titanium values in mixtures of their respective chlorides.
Titaniferous materials are often subjected to chlorination, because it is an efficient and economical way to obtain a high purity titanium source for making titanium alloys, titanium compounds, and especially pigmentary titanium dioxide.
Several processes have been described in the art for the chlorination of titaniferous materials. Such processes generally react a titanium dioxide-providing raw material often containing iron values with a chlorine-providing material and a carbon-containing reductant according to one or both of the following equations: EQU TiO.sub.2 +2CL.sub.2 (g)+C(s).fwdarw.TiCl.sub.4 (g)+CO.sub.2 (g) EQU TiO.sub.2 +2Cl.sub.2 (g)+2C(s).fwdarw.TiCl.sub.4 (g)+2CO(g)
Although the presence of iron is optional in the titanium-providing raw materials, most chlorination processes are effective for concomitantly chlorinating the Ti and Fe values of the feed materials as shown in the following reactions: EQU 2FeTiO.sub.3 +6Cl.sub.2 (g)+3C(s).fwdarw.2TiCl.sub.4 (g)+3CO.sub.2 (g)+2FeCl.sub.2 EQU FeTiO.sub.3 +3Cl.sub.2 (g)+3C(s).fwdarw.TiCl.sub.4 (g)+3CO(g)+FeCl.sub.2
Chlorination reactions are generally carried out at about 1000.degree. C., but can be carried out at any temperature in the range from about 800.degree. C. to about 2000.degree. C., using various carbon reductants and chlorine sources, including chlorine gas and chlorine-providing compounds. The titaniferous materials to be chlorinated can be preformed into briquets or the process can be conducted in a fluid bed using granular materials. When a fluid-bed process is used, generally the chlorine-providing material is supplied to the bottom of the bed and product titanium tetrachloride (TiCl.sub.4) is removed from the top. Fluidization is generally controlled such that the bed remains fluidized and yet fine, solid particulate materials are not carried out with the product.
Selective chlorination processes also exist and are designed to chlorinate only the Ti values or the Fe values of the raw material. A carbon reductant and a chlorine source are used and reaction temperatures are similar to non-selective processes. However, selective processes utilize a chlorine source consisting at least partially of iron chlorides or react the titaniferous raw materials in a dilute phase, or utilize an especially high temperature or a combination of these.
Such titaniferous materials also usually contain vanadium impurities which adversely affect the titanium products produced. For example, pigmentary TiO.sub.2 can only tolerate about 10 ppm. vanadium in the titanium tetrachloride from which it is made without discoloration. Removal of such impurities has heretofore been a complicated and burdensome process because of the similarity between the chemical and physical characteristics of titanium and its compounds and vanadium and its compounds. For example, TiCl.sub.4 melts at -25.degree. C. and boils at 136.4.degree. C. and VCl.sub.4 melts at -28.degree. C. and boils at 148.5.degree. C. This parallelism of properties permeates a comparison of the compounds of these two elements. Therefore, in a conventional chlorination process the vanadium values in a titaniferous material react in substantially the same manner as the titanium values, and their respective chlorinated products have nearly identical chemical and physical properties. Therefore, it is extremely difficult to separate the undesirable chlorinated vanadium values from the desirable titanium values. Fractional distillation, for example, will remove most impurities from TiCl.sub.4, but is ineffective for removing vanadium impurities.
Processes, which are used commercially, remove vanadium impurities from TiCl.sub.4 by refluxing with copper, treating with H.sub.2 S in the presence of a heavy metal soap, or treating with an alkali metal soap or oil to reduce vanadium to a less volatile form, each followed by a further distillation. However, the organic materials used tend to decompose and deposit sticky, adhering coatings on heat exchanger surfaces, pipes, and vessel walls. This causes shutdowns of the process and requires frequent maintenance of the equipment.
A simple, efficient, and economical process has now been discovered for separating the vanadium values from chlorinated titaniferous materials. This process utilizes a high surface area carbon. Said high surface area carbon is reacted with chlorinated titaniferous materials at a temperature in excess of about 500.degree. C. The reaction with this high surface area carbon causes any vanadium values present to be reduced to a less volatile form so that they can be easily removed as a solid from the gaseous or liquid TiCl.sub.4 product.
One advantage of the present process is that it can be performed in existing equipment for the chlorination of titaniferous material with very little modification. Another advantage is that it employs economical raw materials. Still another advantage is that the CO value of the tail gas produced is sufficiently enhanced such that said tail gases will support combustion and can be burned to effect conversion of CO to CO.sub.2 and thus eliminate a pollution problem previously created.